WASHINGTON - DECLARING North Korea a 'grave threat' to the world, President Barack Obama on Tuesday pledged the US and its allies will aggressively enforce fresh international penalties against the nuclear-armed nation and stop rewarding its leaders for repeated provocations.

In a display of unity with South Korea's leader, Mr Obama said the world must break a pattern in which North Korea puts the globe on edge, only to put itself in line for concessions if it holds out long enough.

Push for new policies NORTH Korea provoked that rebuke by conducting its second nuclear test on May 25, following recent missile launches that had already alarmed the world. ... more Tactics won't work WASHINGTON - PRESIDENT Barack Obama and South Korean President Lee Myung-bak on Tuesday forged a united front against North Korea, saying the hermit nation must abandon its nuclear weapons program and will not be rewarded for provoking a crisis. 'There's been a pattern in the past where North Korea behaves in a belligerent fashion and if it waits long enough is then rewarded with food stuffs and fuel and concessionary loans and a whole range of benefits,' Mr Obama said. ... more

'We are more than willing to engage in negotiations to get North Korea on a path of peaceful coexistence with its neighbors, and we want to encourage their prosperity,' Mr Obama said at the White House alongside South Korean President Lee Myung-bak. 'But belligerent, provocative behavior that threatens neighbors will be met with significant and serious enforcement of sanctions that are in place.'

Mr Obama's comments came at a time of intensifying concern, with the North stepping up its bomb-making activities and threatening war against any country that blockades its ships. Pentagon officials warned on Tuesday that North Korea's missiles could strike the US within three years if its weapons growth goes unchecked.

Emboldened by fresh assurances of protection by the United States, Mr Lee went even further in warning that North Korea's tactics will not be tolerated. Asked if he felt his country was under the threat of attack from the North, Mr Lee said his country's alliance with the US will 'prevent anything from happening'.

He said of the North Koreans, 'They will think twice about taking any measures that they will regret.'

Defiantly pursuing its nuclear ambitions, North Korea has posed a major foreign policy challenge for Mr Obama. However, the new president has found support from the international community, including a swift resolution of sanctions imposed by the UN Security Council just last week.

The new punishments toughen an arms embargo against North Korea and authorise ship searches in an attempt to thwart the Koreans' nuclear and ballistic missile programmes. The UN, however, did not authorise military force to enforce the measures. -- AP